1. Field of the Invention
The present invention relates to an x-ray computed tomography device and method, and more particularly to a helically scanning x-ray computed tomography device and method where the effective slice thickness may be adjusted.
2. Discussion of the Background
As shown in FIG. 6, an x-ray computed tomography device (hereinafter, it is referred as "x-ray CT device") comprises an x-ray tube 42 for irradiating a beam of x-rays 41 and a detector 46 for detecting x-rays passed through a detected object, such as patient 43, wherein the detector 46 is positioned at a side opposite to the x-ray tube 42 with respect to the detected object and a tomography image can be obtained by rotating the x-ray tube 42 integrally with the detector 46 around the detected object (shown by arrow 45) in order to scan the detected object and reconstruct the projected data.
Such a tomographic image obtained by an x-ray CT device contains area information of the x-ray beam 41 having a predetermined width along a body longitudinal direction of the detected object, wherein the x-ray beam 41 passes through the detected body and is irradiated on detector 46 with a predetermined width along the same direction. The area is called the slice thickness and is indicated as 44. In general, slice thickness 44 is determined by a width of the x-ray beam 41 at a rotational center 47. The slice thickness is also determined by setting an x-ray optical mechanism including a beam trimming machine 40 located between x-ray tube 42 and the detected object and a beam trimming machine 48 located between the detected object and detector 46. In other words, a desirable slice thickness can be set by changing the settings of the x-ray optical mechanism. In an x-ray CT device utilized in the medical industry, a desirable slice thickness can be selected from a plurality of predetermined values (for example, one value, such as 2 mm, is selected from a set of values, i.e., 1 mm, 2 mm, 5 mm, 7 mm and 10 mm) by considering the desired diagnostic purpose and scanning period. Then, the x-ray optical mechanism is set to match the selected slice thickness.
As shown in FIG. 7, in a conventional x-ray CT device employing a helical scan method, a rotatable frame on which an x-ray tube 42 and a detector 46 are mounted is continuously rotated by connecting with a frame fixed portion through a slipping device (not shown). A bed on which a detected object is disposed is moved along a body longitudinal direction in order to scan helically, indicated as 49, and then projection data of the scanned detected object is obtained rapidly. In such a conventional x-ray CT device employing a helical scan method, actual data is interpolation processed in order to reconstruct a tomographic image at a slice portion. Therefore, an effective slice thickness of the tomographic image obtained by reconstruction (an effective slice thickness can be determined by a known method) is likely to be thicker than a thickness selected by the x-ray optical mechanism. The thicker an effective slice thickness becomes, the lower the spatial resolution capability becomes.